1. Field of the Invention
The present invention relates to a double insulation structure of an electric tool and its insulating method and, more particularly, to the safety of the operator from the electric leakage and the size reduction of the aluminum outer casing as the grip portion when the metal outer casing is used in the portable electric disk grinder that is widely employed in grinding/polishing operations of the workpiece.
2. Description of the Related Art
As the electric tool in the prior art, the portable electric disk grinder employed widely in grinding/polishing operations of the workpiece, for example, will be explained with reference to FIG. 14 to FIG. 19 hereunder. FIG. 14 is a partially-omitted longitudinally-sectioned side view showing a double insulation type portable electric disk grinder in the prior art, which has an outer casing 15 that is equipped with a motor 2 as a driving source to support rotatably a rotor 2b of the motor 2. FIG. 15 is a sectional view taken along an H—H line in FIG. 14. FIG. 16 is a partially-omitted longitudinally-sectioned side view showing another double insulation type portable electric disk grinder in the prior art, which has a metal bushing 16 in the outer casing 15 that is equipped with the motor 2 as the driving source to support rotatably the rotor 2b of the motor 2. FIG. 17 is a sectional view taken along an I—I line in FIG. 16. FIG. 18 is a partially-omitted longitudinally-sectioned side view showing a single insulation type portable electric disk grinder in the prior art, which has an outer casing 17 that is equipped with the motor 2 as the driving source to support rotatably the rotor 2b of the motor 2. FIG. 19 is a sectional view taken along a J—J line in FIG. 18.
A double insulation type portable electric disk grinder shown in FIG. 14 and FIG. 15 comprises an outer casing 15 formed of insulating resin, a motor 2 including a stator 2a and a rotor 2b installed in this outer casing 15, a bearing 3 for supporting rotatably an end portion of the rotor of the motor 2, a rotor supporting portion 15a formed in the outer casing 15 to support this bearing 3, a carbon brush 4 that contacts electrically a commutator 2c of the rotor 2b, a carbon brush holder 5 fixed to the outer casing 15 to hold the carbon brush 4, a power transmitting gear 6 provided on the opposite side to the commutator 2c of the rotor 2b, and a tip tool such as a grindstone, a disk, or the like connected to an output shaft (not shown) to receive a rotary power from the power transmitting gear 6 via the output shaft. Also, in the above configuration, the reinforced insulation is applied to the carbon brush holder 5 and also the double insulation is applied to the rotor 2b. In addition, the double insulation is achieved by the supplementary insulation of the outer casing 15 in addition to the functional insulation that is applied to the stator 2a and a power supply line 7.
In such configuration, the operator supplies the power to the motor 2 by operating a switch (not shown) to rotate the rotor 2b, and then transmits the rotary power via the power transmitting gear 6 and the output shaft to carry out the grinding/polishing operations of the workpiece.
In this case, since the outer casing 15 that is gripped by the operator is formed of the insulating resin material, the double insulation can be easily achieved and also the creeping distance to cause a short circuit can be prolonged even if the metal powders (dusts) are produced continuously when the conductive material such as the metal is worked. Therefore, the safety of the operator from the electric leakage can be assured.
However, according to the above configuration, the rotor supporting portion 15a is heated by the deterioration of the lubricating oil, the fracture, etc. of the bearing 3 in addition to the heat that is generated in the charging portions of the stator 2a and the rotor 2b and the yoke. Therefore, there is the drawback that, when such increase in the temperature exceeds the heat resisting temperature of the outer casing 15, the rotor supporting portion 15a and its periphery of the outer casing 15 is thermally deformed. Also, the high heat generated due to the degradation of rectification is transmitted to the outer casing 15 from the carbon brush 4 via the carbon brush holder 5. Therefore, there is the drawback that, when such increase in the temperature exceeds the heat resisting temperature of the outer casing 15, the outer casing 15 around the carbon brush holder 5 is thermally deformed. In this manner, if the outer casing 15 is thermally deformed, there are the problems that the rotor supporting portion 15a of the outer casing 15 cannot support firmly the bearing 3, which supports rotatably the rotor 2b, to cause the reduction in the performance of the motor 2 and that the outer casing 15 is ready to deform or break by the external impact generated when the above grinder is dropped down by mistake, etc. In this case, in order to overcome these problems, such outer casing 15 must be replaced with a new outer casing 15.
Also, a double insulation type portable electric disk grinder shown in FIG. 16 and FIG. 17 comprises the outer casing 15 formed of the insulating resin, the motor 2 including the stator 2a and the rotor 2b installed in this outer casing 15, the bearing 3 for supporting rotatably the end portion of the rotor of the motor 2, the rotor supporting portion 15a formed in the outer casing 15 to support this bearing 3, a metal bushing 16 provided between the rotor supporting portion 15a and the bearing 3, the carbon brush 4 that contacts electrically the commutator 2c of the rotor 2b, the carbon brush holder 5 fixed to the outer casing 15 to hold the carbon brush 4, the power transmitting gear 6 provided on the opposite side to the commutator 2c of the rotor 2b, and the tip tool such as the grindstone, the disk, or the like connected to the output shaft (not shown) to receive the rotary power from the power transmitting gear 6 via the output shaft. Also, in the above configuration, the reinforced insulation is applied to the carbon brush holder 5 and also the double insulation is applied to the rotor 2b. In addition, the double insulation is achieved by the supplementary insulation of the outer casing 15 in addition to the functional insulation that is applied to the stator 2a and a power supply line 7. Further, the metal bushing 16 that can radiate the heat, which is generated in the charging portions of the stator 2a and the rotor 2b and the yoke, as well as the heat, which is generated by the deterioration of the lubricating oil, the fracture, etc. of the bearing 3, to some extent is provided. As a result, the thermal deformation around the rotor supporting portion 15a of the outer casing 15 can be suppressed.
In such configuration, the operator supplies the power to the motor 2 by operating a switch (not shown) to rotate the rotor 2b, and then transmits the rotary power via the power transmitting gear 6 and the output shaft to carry out the grinding/polishing operations of the workpiece.
In this case, since the outer casing 15 that is gripped by the operator is formed of the insulating resin material, the double insulation can be easily achieved and also the creeping distance to cause a short circuit can be prolonged even if the metal powders (dusts) are produced continuously when the conductive material such as the metal is worked. Therefore, the safety of the operator from the electric leakage can be assured.
However, according to the above configuration, there is the similar problem that, when the heat that exceeds an amount of heat radiation of the metal bushing 16 is generated, the increase in the temperature exceeds the heat resisting temperature of the insulating resin and thus the thermal deformation of the outer casing 15 is brought about.
Also, a single insulation type portable electric disk grinder shown in FIG. 18 and FIG. 19 comprises an outer casing 17 formed of conductive material such as metal (aluminum), or the like, the motor 2 including the stator 2a and the rotor 2b installed in this outer casing 17, the bearing 3 for supporting rotatably the end portion of the rotor of the motor 2, a rotor supporting portion 17a formed in the outer casing 17 to support this bearing 3, the carbon brush 4 that contacts electrically the commutator 2c of the rotor 2b, the carbon brush holder 5 fixed to the outer casing 17 to hold the carbon brush 4, the power transmitting gear 6 provided on the opposite side to the rotor 2b, and the tip tool such as the grindstone, the disk, or the like connected to the output shaft (not shown) to receive the rotary power from the power transmitting gear 6 via the output shaft. Also, in the above configuration, the reinforced insulation is applied to the carbon brush holder 5 and also the single insulation is achieved by the functional insulation that is applied to the stator 2a, the rotor 2b, and the power supply line 7.
In such configuration, the operator supplies the power to the motor 2 by operating a switch (not shown) to rotate the rotor 2b, and then transmits the rotary power via the power transmitting gear 6 and the output shaft to carry out the grinding/polishing operations of the workpiece.
Thus, since the outer casing 17 that is gripped by the operator is formed of the conductive material such as the metal, the heat can be radiated from a surface of this outer casing 17 even when the rotor supporting portion 17a is heated by the deterioration of the lubricating oil, the fracture, etc. of the bearing 3 in addition to the heat that is generated in the charging portions of the stator 2a and the rotor 2b and the yoke, or the high heat generated due to the degradation of rectification is transmitted to the outer casing 17 from the carbon brush 4 via the carbon brush holder 5. Therefore, the thermal deformation of the outer casing 17 can be suppressed. Accordingly, the bearing 3 that supports rotatably the rotor 2b can be held without fail by the rotor supporting portion 17a of the outer casing 17, and also the outer casing 17 becomes hard to deform or break by the external impact generated when the above grinder is dropped down in error, etc.
However, according to the above configuration, there is the problem that, if the metal powders are produced continuously when the conductive material such as the metal is worked, the safety from the electric leakage is lowered since the creeping distance to cause the short circuit is short.
In the configurations of the double insulation type portable electric disk grinders shown in FIG. 14 to FIG. 17, as described above, the rotor supporting portion is heated by the deterioration of the lubricating oil, the fracture, etc. of the bearing in addition to the heat that is generated in the charging portions of the stator and the rotor and the yoke. Therefore, there is the drawback that, when such increase in the temperature exceeds the heat resisting temperature of the outer casing, the rotor supporting portion and its periphery of the outer casing is thermally deformed. Also, the high heat generated due to the degradation of rectification is transmitted to the outer casing from the carbon brush via the carbon brush holder. Therefore, there is the drawback that, when such increase in the temperature exceeds the heat resisting temperature of the outer casing, the outer casing around the carbon brush holder is thermally deformed. In this way, if the outer casing is thermally deformed, there are the problems that the rotor supporting portion of the outer casing cannot support firmly the bearing, which supports rotatably the rotor, to cause the reduction in the performance of the motor and that the outer casing is ready to deform or break by the external impact generated when the above grinder is dropped down by mistake, etc. In this case, since such outer casing must be replaced with the new outer casing to overcome these problems, there are caused the drawbacks that the burden is imposed on the operator in respect of the operation or the expense and that the effective utilization of the resource cannot be attained.
Also, according to the configuration of the single insulation type portable electric disk grinder shown in FIG. 18 and FIG. 19, there is the problem that, if the metal powders are produced continuously when the conductive material such as the metal is worked, as described above, the safety from the electric leakage is lowered since the creeping distance to cause the short circuit is short.